Move BRIN page type to page's last two bytes

Alvaro Herrera <alvherre@alvh.no-ip.org>

Commit: e491bd2ee34860b14ff18abc5602f9aa5b197a2d
Author: Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2015-03-10T15:27:15Z
Releases: 9.5.0
Move BRIN page type to page's last two bytes

... which is the usual convention among AMs, so that pg_filedump and
similar utilities can tell apart pages of different AMs.  It was also
the intent of the original code, but I failed to realize that alignment
considerations would move the whole thing to the previous-to-last word
in the page.

The new definition of the associated macro makes surrounding code a bit
leaner, too.

Per note from Heikki at
http://www.postgresql.org/message-id/546A16EF.9070005@vmware.com

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